Diffuse node compability for glTF exporter
Just setting metallic to black(0) and Roughness to white(255) should solve the problem of getting a diffuse shader.

I think that your shader is diffuse but you cant see the colors when you export it out. I had the same problem, I solved it by using the Emission node packed with the two nodes you mentioned.



All those texts are using emissive nodes.

Also applying material on text objects doesnt work. You have to break them to mesh, draw out UV map and then apply texture(also using emissive map).